Digital Subscriber Line (DSL) technology is a high speed transmission technology for transmitting data via telephone twisted pair, i.e., Unshielded Twisted Pair (UTP). DSL includes Asymmetrical Digital Subscriber Line (ADSL), Very-high-bit-rate Digital Subscriber Line (VDSL), ISDN Digital Subscriber Line (IDSL) and Single-pair High-bit-rate Digital Subscriber Line (SHDSL), etc.
In various DSL technologies (xDSL), except for the DSL performing transmission on baseband such as IDSL, SHDSL, the DSL performing transmission on passband uses a frequency division multiplex technology, so that the DSL and the Plain Old Telephone Service (POTS) may coexist on the same twisted pair, where the DSL occupies the high frequency band, and the POTS occupies the baseband part below 4 KHz. The POTS signal and the DSL signal are split or combined via a splitter. The xDSL performing transmission on passband employs a Discrete Multi-Tone modulation (DMT) technology for modulation and demodulation. A system that provides multi-channel DSL access is referred to as a DSL Access Multiplexer (DSLAM). FIG. 1 is a schematic diagram showing a reference model of an xDSL system, where DSLAM 120 includes an office-end transceiver unit 121 and a splitter 122. In the uplink direction, the office-end transceiver unit 121 receives a DSL signal from a computer 110 and amplifies the received signal, and then sends the processed DSL signal to the splitter 122; the splitter 122 integrates the DSL signal from the office-end transceiver unit 121 and the POTS signal from a telephone terminal 130; the integrated signal is transmitted via a multi-channel UTP 140, and received by a splitter 151 in the remote DSLAM 150; the splitter 151 splits the received signal, sends the POTS signal in the received signal to the Public Switched Telephone Network (PSTN) 160, and sends the DSL signal in the received signal to a transceiver unit 152 of a DSLAM 150; the transceiver unit 152 amplifies the received signal and then sends the amplified signal to a Network Management System (NMS) 170. In the downlink direction of the signal, the signal is transmitted in a reverse order with respect to the above order.
As the frequency band used in xDSL technology is raised, the problem of crosstalk, especially the high frequency band crosstalk, becomes more and more outstanding with each passing day. FIG. 2A and FIG. 2B are schematic diagrams of a near-end crosstalk and a remote-end crosstalk in xDSL, respectively. As shown in FIG. 2A, the near-end crosstalk (NEXT) refers to the crosstalk between the PORT 1 and the adjacent PORT2 in DSLAM 210 as well as the crosstalk between adjacent remote-terminal unit (RTU, in the embodiments of the invention, it mainly refers to the DSL modem) 1 and RTU2. As shown in FIG. 2B, the remote-end crosstalk (FEXT) refers to the crosstalk between the PORT1 in the DSLAM 220 and the remote RTU2 as well as the crosstalk between the PORT2 and the remote RTU1. Because the xDSL uplink and downlink channels employ the frequency division multiplex technology, the NEXT does not generate a severe harm to the performance of the system. However, the FEXT may seriously influence the transmission performance of the line. When users of a plurality of channels in a bundle of cables request to activate the xDSL service, the FEXT may cause the low rate and unstable performance of some lines, or it may even give rise to the problem that the xDSL service cannot be activated. As a result, a low line activation rate of DSLAM may be caused.
The high frequency attenuation is another main factor that influences the transmission performance of the xDSL. FIG. 3 and FIG. 4 illustrate the attenuation characteristics of American Wire Gauge (AWG) 26 at a length of 300 m and 400 m, respectively. In FIG. 3 and FIG. 4, the longitudinal axis represents the signal amplitude, the horizontal axis represents the frequency, and each frequency interval represents 4.3125 kHz. For example, 4000 represents that the practical frequency is 4000*4.3125 kHz=17.25 Mhz. It can be seen from these two drawings that the higher the frequency is, the greater the attenuation of the signal is; and the longer the line is, the greater the attenuation of the signal is.
In view of the current xDSL technical standard, theoretically, the VDSL2 may provide a maximum uplink and downlink symmetrical rate of 100 Mbps. However, due to the influence of the above remote-end crosstalk and the signal high frequency attenuation, the transmission rate in practical disposition process is much lower than 100 Mbps. Even if a symmetrical 100 Mbps rate is provided, the transmission distance that can be achieved is only about 300 m. When a client demands a higher rate or a larger service radius, the VDSL2 technology would be helpless.
To meet the requirements of clients on higher rate or larger service radius, a technology for common transmission over multiple line pairs takes the stage. The technology for common transmission over multiple line pairs, in which multiple pairs of subscriber lines are used as physical transmission media simultaneously, may solve the problem that a single pair of DSLs cannot provide a higher speed.
The bonding technology is an earlier technology for common transmission over multiple line pairs. However, because the high frequency band is influenced by the remote-end crosstalk, the integrated performance of such technology is far from the linear superimpose of the performance of each line. The Multiple Input Multiple Output (MIMO) technology solves the problem that the bonding technology is influenced by the remote-end crosstalk.
The operating principle of the MIMO technology is as follows:
FIG. 5 is a schematic diagram of the MIMO technology, where transmitters T1, T2, T3 and T4 are connected with receivers R1, R2, R3 and R4 respectively, in other words, four pairs of subscriber lines are used for transmitting data simultaneously. X1, X2, X3 and X4 are signals sent by the transmitters T1, T2, T3 and T4 respectively; Y1, Y2, Y3 and Y4 are signals received by the receivers R1, R2, R3 and R4 respectively; H1,1, H2,2, H3,3H4,4 are transmission functions of four transmission lines respectively.
Hn,n represents the transmission function of line n, Xn represents the signal sent by transmitter n, Yn represents the signal received by receiver n. Because of the crosstalk, the signal Y1 received by the receiver R1 not only includes H1,1·X1, but also includes signal components generated on Y1 by signals sent by T2, T3 and T4 due to the remote-end crosstalk, as represented by the following formula:Y=H·X+N  Formula (1)where Y=[Y1,Y2,Y3,Y4], X=[X1,X2,X3,X4] and N=[N1,N2,N3,N4] are all 1*4 vectors, and
  H  =      [                                        H                          1              ,              1                                                            H                          1              ,              2                                                            H                          1              ,              3                                                            H                          1              ,              4                                                                        H                          2              ,              1                                                            H                          2              ,              2                                                            H                          2              ,              3                                                            H                          2              ,              4                                                                        H                          3              ,              1                                                            H                          3              ,              2                                                            H                          3              ,              3                                                            H                          3              ,              4                                                                        H                          4              ,              1                                                            H                          4              ,              2                                                            H                          4              ,              3                                                            H                          4              ,              4                                            ]  is a 4*4 matrix. Hn,m represents the remote-end crosstalk function of Line m to Line n when m is not equal to n. Therefore, according to Formula (1), the signal received by the receiver R1 is
                    Y        ⁢        1            =                                    H                          1              ,              1                                ·                      X            ⁢            1                          +                  N          ⁢          1                +                              ∑                          n              =              2                        4                    ⁢                                          ⁢                                    H                              1                ,                n                                      ·            Xn                                ,    ⁢        where the summation component on the right side of the above formula represents the sum of all the crosstalks. If the bonding technology is employed, the summation term may only be treated as a noise. However, it can be seen from this formula that there includes information of the signal sent. The MIMO technology just utilizes this feature and employs a signal joint processing mode to eliminate the crosstalk using this signal. In other words, both sides of Formula (1) are multiplied by H−1 simultaneously: H−1Y=H−1H·X+H−1N. Therefore, the final received signal is Y′=X+H−1N. It can be seen that the signal received is only related to the sent signal itself. In other words, the crosstalk is eliminated.
Practically, the MIMO technology further includes some other algorithms, and the influence of crosstalk is eliminated theoretically, thus the performance is higher than the bonding technology.
In view of the current DSL technology, the MIMO is the DSL technology with the optimal performance. However, in view of the modulation mode, the MIMO technology still uses the currently popular Orthogonal Frequency Divided Multiple (OFDM) mode. In other words, the transmission capacity estimation method of each pair of lines is still similar to the estimation method of VDSL2, and the advantages of joint processing of multiple line pairs are not given a full play.
In the MIMO technology, the communication capacity estimation of each pair of lines is as follows:
In the field of communications, there exists a well-known Shannon Theorem about line communication capacity estimation, the formula of which is referred to as Shannon channel capacity formula:
                    C        =                              W            ·                                          log                2                            ⁡                              (                                  1                  +                                      S                    N                                                  )                                              ⁢                                          ⁢                      (                          bit              ⁢                              /                            ⁢              second                        )                                              Formula        ⁢                                  ⁢                  (          2          )                    where C represents the channel capacity; S represents the signal amplitude of the receiving end; N represents the noise amplitude; and W represents the signal bandwidth.
It can be seen from Shannon channel capacity formula that the wider the signal bandwidth is, the greater the channel capacity is; the greater the signal amplitude is, the greater the channel capacity is; and the smaller the noise is, the greater the channel capacity is.
In the MIMO technology, because factors such as spectrum compatibility and analog devices are considered, a maximum transmitted power spectrum density (PSD(f)) is usually defined on the transmitting end. Thus the maximum value of S in Shannon channel capacity Formula (2) is determined, and this maximum value is equal to the maximum transmitted power value subtracting the attenuation value. Therefore, the factors that determine MIMO communication capacity only include the noise and the signal bandwidth, where the noise mainly includes the white noise and the crosstalk, etc.
Because the crosstalk can be eliminated by the MIMO technology, only the white noise will be considered below. In FIG. 6, the longitudinal axis represents the signal amplitude, the horizontal axis represents the frequency, and each frequency interval represents 4.3125 kHz. The curve 201 represents the power spectrum density of a flat transmitted power spectrum density on the receiving end after line attenuation. The curve 202 represents the white noise of the receiving end. It can be seen from FIG. 6 that the point 204 where the signal-to-noise ratio (SNR) is greater than zero lies about the frequency 4400*4.3125 kHz. In the OFDM modulation technology, because data can only be carried when the SNR is greater than 3 dB, the frequency points that may be used for data transmission are frequencies below point 203, i.e., the frequency segments in the range of 0-4000*4.3125 kHz. For the frequencies between the point 203 and the point 204, although the SNR is greater than zero, they cannot be used for carrying data. This is a great waste. Therefore, in the MIMO technology, the advantages of joint processing of multiple line pairs are not given a full play.
Additionally, for different practical application environments, the noise of each pair of lines and the channel attenuation are different. As shown in FIG. 7, it shows a schematic diagram of the characteristics of the SNR with respect to frequency of four pairs of different lines. There are totally 8 frequency segments (from 801 to 808) that cannot carry one unit of bit. The SNRs corresponding to these frequency segments are all less than 0 dB, and they cannot meet the condition to carry data in the modulation technology. Therefore, these frequency segments are not utilized either, thus the channel resources are wasted.